Bore seem to be a bit small on my 45-70

[corbinace]

The barrel is a modern 1.120 OD octagon barrel on a 'new to me" Remington Rolling Block. No markings of any sort on barrel. 1:20 twist. It has eight lands and grooves.

My measurements come up to .457" and .445"

I only drove one slug through it but I did measure the barrel again at the muzzle, because I could not believe the slug. Measuring at the muzzle with dial calipers shows .446". It does have relatively deep sharp edges on the grooves/lands. Slug started at about .470".

Do I need to get my eyeballs re-calibrated or is there a chance that this could be right?

My only other experience with this cartridge is a trapdoor with three lands and grooves, so it is a bit tough to measure, and not nearly so sharp.

I know, get out the cerrosafe.:groner: I just thought I would check to see if maybe I was reading it right.

Thanks, Tim.

[Shiloh]

Calipers or micrometer?? Try it with a micrometer. My cheap Chinese mike is as accurate as a mid price range Mitutoyo mike.
Calipers can be off .001 or more. Many bores are oversize. You can always size down. You have the advantage of using a softer, purer lead.

Shiloh

[NSB]

I've slugged three of my 45-70 bbls and they all came out at .4575". Since you're reporting the results in three decimal places, I'm going to assume your calipers don't measure out to four decimal places. If that's the case, you're not going to get an accurate measurement. Your could be at .4571 or .4579. You need the fourth decimal place. I worked in a gage lab for a while and I calibrated thousands of calipers and micrometers. There's no truth at all in the statement that calipers can be off by a thousandth. They're as accurate as micrometers if used in the correct application.

[44man]

Cerrosafe will never give the right measurements no matter the time. I have fooled with the stuff for years and found a pure slug is the best way.
Dial calipers can be accurate but my old ones need magnifying glasses to read. The mike works best. Need four places too.

[TXGunNut]

Did you feel any tight spots while slugging the bore?

[NSB]

Cerrosafe will never give the right measurements no matter the time. I have fooled with the stuff for years and found a pure slug is the best way.
Dial calipers can be accurate but my old ones need magnifying glasses to read. The mike works best. Need four places too.
Tell me about the "old eyes". I've gone to digital calipers and mics and use gage blocks to check them with periodically. Those "little lines" are nearly invisible at this stage in my life.

[Huvius]

Are you going to shoot BP, smokeless or both?
I have had great success measuring the bore with pin gauges only for my BP rifles since I use boolits sized to bore (or a 'thou bigger). Never felt that the groove depth mattered much if you are shooting soft lead.
Otherwise, for smokeless, it is 1-2 'thou bigger than groove if your chamber will allow a boolit that big. Some just go with a boolit that will fit snugly in a fired case assuming your chamber leade is appropriate for shooting cast. I think that means a more gentle slope to the start of the rifling (may be wrong on that).

[Tatume]

Groove diameter of 0.457" is common.

[corbinace]

Thank you for your replies Gentlemen,

I guess I did not accurately state my question. I am wondering about the bore size of 0.445-.446. That seems to be a bit small to my uneducated way of thinking. I expected to see 0.450" on a 45 caliber bore.

I do have a Micrometer for measuring the slug. It will work fine for measuring the high points on the slug (the groove of the barrel) not so much on the lands. There are eight of them and they are only .080" wide on the slug, so they cannot be measured by .270" wide button and shaft of my mic.

The 30+ year old dial calipers are of good quality and checked periodically with a one inch standard and zeroed each time I touch them.

My cheapo digital calipers measured the same and break the forth place into .0005 increments.

TX, no tight spots, it felt really nice.

Unfortunately, I do not have access to pin gauges. I intend to shoot both BP and smokeless. This rifle is to be a BP learning tool for me.

Since there seamed to be no concern with the smaller bore dimension and I can as Shiloh and Huvius say, use a softer alloy, I guess I must be golden.

One thing that I did think about while laying in bed, was the cleanliness of the bore. Maybe there could be enough buildup of copper or something to reduce the size. I will do a bit of cleaning on it and see.

Thank you again for your consideration of the question, it is appreciated.

Tim